Could conceivably be a breach of the peace if the recording came to the notice of the usual occupant of the seat and she was alarmed. More likely to be a disciplinary matter for the employer.

The usual occupant seeing the recording is irrelevant, the employee can still be charged with breach of the peace - all that has to be established is that anybody who would see that kind of action could be shocked or alarmed. In this circumstance if the police wanted to proceed then they would likely be convicted, there isn't really much of a defence - banter or not - as somebody could still be alarmed by it. (I'm not passing moral judgement btw, just conveying the legalities! )